1. Field of the Invention
The present disclosure relates to a prism sheet, and more particularly, to a prism sheet with improved optical efficiency and a liquid crystal display device having the same.
2. Background of the Related Art
Display devices act as visual information output media that provide various types of images to users. Consumers require that display devices meet certain requirements, such as exhibiting low power consumption, having a thin form factor, being lightweight, and being capable of outputting high resolution images.
Display devices may be classified into an emissive type and a transmissive type. The spontaneous light emitting (emissive) type includes Cathode Ray Tube (CRT) devices, Electro Luminescence (EL) devices, Organic Light Emitting Diode (OLED) devices, Vacuum Fluorescent Display (VFD) devices, a Field Emission Display (FED) devices, Plasma Display Panel (PDP) devices, and the like. The non-spontaneous light emitting (transmissive) type includes Liquid Crystal Display (LCD) devices and the like.
LCD devices operate to output images by using the optical anisotropy characteristics of liquid crystals. Such LCD devices have been widely used as a display device because of relatively low heat generation, excellent visibility characteristics, and low average power consumption compared to conventional cathode-ray tube devices. Hereinafter, a typical LCD device will be described in more detail.
In general, the LCD device may display a desired image by controlling a light transmittance of pixels by individually supplying a data signal corresponding to image information to each of the pixels. The LCD device includes a liquid crystal panel where pixels are arranged in a matrix form, a driving unit configured to drive the pixels, and a backlight configured to supply light to the liquid crystal panel.
FIG. 1 is a schematic exploded perspective view showing a structure of a related art LCD device.
Referring to FIG. 1, the related art LCD device includes a liquid crystal panel 10 configured to output images via pixels arranged in a matrix form, a driving unit configured to drive the pixels, a backlight 40 mounted on a rear surface of the liquid crystal panel 10 and configured to supply light to a front surface of the liquid crystal panel 10, and a lower cover 50 configured to accommodate the liquid crystal panel 10 and the backlight 40 therein.
The liquid crystal panel 10 includes a color filter substrate 5, a TFT array substrate 15, and a liquid crystal layer which is formed in a cell gap between the color filter substrate 5 and the TFT array substrate 15. The color filter substrate 5 and the TFT array substrate 15, disposed to face each other, are attached to each other such that the cell gap is maintained.
The liquid crystal panel 10 further includes a common electrode and a pixel electrode configured to apply an electric field to the liquid crystal layer.
When a voltage of a data signal applied to the pixel electrode is controlled in a state where such voltage is applied to the common electrode, the liquid crystal panel 10 operates to display text or images by passing or blocking light for each pixel by rotating liquid crystals of the liquid crystal layer by dielectric anisotropy according to the electric field created between the common electrode and the pixel electrode.
A switching device such as a thin film transistor (TFT) is separately provided at each pixel such that the voltage of the data signal applied to the pixel electrode is controlled per pixel.
Upper and lower polarizing plates are attached on an outer surface of the liquid crystal panel 10. In this case, the lower polarizing plate acts to polarize light which has passed through the backlight 40, and the upper polarizing plate acts to polarize light which has passed through the liquid crystal panel 10.
The backlight 40 will be explained more specifically. A light source, such as a light emitting diode (LED) assembly 30 configured to generate light is installed at one side of a light guide plate 42, and a reflection plate 41 is installed on a rear surface of the light guide plate 42. The LED assembly 30 includes an LED array 31 and an LED printed circuit board (PCB) 32 having circuits configured to drive the LED array 31.
Light emitted from the LED array 31 is incident onto a side surface of the light guide plate 42 made of a transparent material. The light transmitted to the rear surface of the light guide plate 42 is reflected toward optical sheets 20 disposed on an upper surface of the light guide plate 42 by the reflection plate 41. In such manner, loss of light is reduced and uniformity of light is enhanced.
The liquid crystal panel 10 is mounted on an upper portion of the backlight 40 by using a guide panel 45. A lower cover 50 is coupled to a lower portion of the backlight 40, thereby constituting a LCD device.
The optical sheets 20 may include upper and lower diffusion sheets 21 and 22, and upper and lower prism sheets 23 and 24. The upper and lower prism sheets 23 and 24 collect light transmitted from the lower diffusion sheet 22, and make the light be uniformly incident onto a front surface of the liquid crystal panel 10.
FIG. 2 is a sectional view showing the refraction characteristics of the light passing through the prism sheet.
Referring to FIG. 2, the prism sheet 23 includes a flat base film 23a and a plurality of prism patterns 23c arranged on the base film 23a in a particular direction. The prism patterns 23c are formed of an acrylic resin having a high refractive index.
According to Snell's law, some light passing through the prism patterns 23c is refracted and directed toward an observer (region ‘A’), while some light is recycled by total reflection.
Part of the light transmitted through the air through the prism patterns 23c is refracted along a thread of the prism patterns 23c by Snell's law to thus be directed toward the observer (region ‘A’). In addition, although part of the light is recycled by total reflection, the light incident at an angle more than or less than a certain angle is refracted toward an invalid area (region ‘B or C’) and is thus lost.
The light refracted toward the invalid area is light that is not directed toward an observer located perpendicularly to the prism sheet 23. The light refracted to the invalid area may occur by total reflection due to a difference of refractive indexes between two media (e.g., the prism patterns 23c and air).